Adjustable electric resistor



July 30, 1968 G. BURNS 3,395,376

ADJUSTABLE ELECTRIC RESISTOR Original Filed March 18, 1965 2 Sheets-Sheet 1 m Q R w) m \Q s Q INVENTOR.

Lawrence (21 Bur/76 July 30, 1968 L. G. BURNS ADJUSTABLE ELECTRIC RESISTOR 2 SheetsSheet 2 Original Filed March 18, 1965 INVENTOR.

Q B r/73 Wm ATTORNEYS 3,395,376 ADJUSTABLE ELECTRIC RESISTOR Lawrence G. Burns, Bristol, Conn., assignor to The Superior Electric Company, Bristol, Conn., a corporation of Connecticut Continuation of application Ser. No. 440,896, Mar. 18, 1965. This application Dec. 14, 1967, Ser. No. 690,686 3 Claims. (Cl. 338-176) ABSTRACT OF THE DISCLOSURE An adjustable resistor includes a helical resistance element wound on a mandrel. The mandrel is an elongated cylindrical shell which has a longitudinal gap formed therethrough. The resistor element is wound to continually compress the mandrel (partially closing the gap) whereby the mandrel will expand or be further compressed in response to temperature changes, depending on the relative coefficients of expansion of the mandrel and resistance element. The inherent resiliency of the mandrel serves to bias the cylinder to its expanded state and alternatively, a spring is supported across the gap to supplement the biasing effect.

This application is a continuation of application Serial No. 440,896.

The present invention relates to an adjustable electric resistor and more particularly to such a resistor which is capable of controlling relatively large amounts of electrical power.

While there are many types of adjustable resistances, a common type that is used to control relatively large amounts of electrical power is generally termed a slide wire resistor. The present invention is of this type and as with most other resistances of the same type employs an elongate element on which a helical coil of resistance wire is wound. A slider is mounted on a frame for linear movement along the elongate element and carries contacts that are adapted to engage at least one of the turns of the coil. By controlling the position of the slider on the coil, the amount of resistance in a circuit can thereby be controlled.

While such heretofore devices have been found to operate, difliculties have arisen. Thus, for example, as the elongate element has been made of elactrically insulating material, it has also been a poor heat conductor and the heat generated in a portion of resistance during use remains in the area producing a concentration of heat that raised the temperature in that portion to an undesirable degree. Additionally, the changes in temperature of the resistor also produced a change in the dimensions of the elongate element and the coil, and as the dimensional changes were different, they caused the turns to become loose on the element. A resistor having loose turns is undesirable because it may cause poor contact between it and the contacts of the slider, produce an unsmooth surface on which the slider has difficulty in moving or cause the spaced apart turns to engage to partially or fully shortcircuit the engaging turns.

It is accordingly an object of the present invention to provide an adjust-able electric resistor of the slide wire type which prevents the turns at all temperatures of use of the resistor to thereby assure good electrical operations and ease of movement of the movable slider even over extended periods of use with wide temperature variations of the resistor.

Another object of the present invention is to provide an electrical resistor of the above type in which the elongated element over which the resistance wire is wound is made to continuously maintain outward tension on the United States Patent 3,395,376 Patented July 30, 1968 ice turns of the wire at all temperatures of the resistor to maintain the turns in position.

A further object of the present invention is to provide an electric resistor of the slide wire type in which concentrations of heat are prevented by the heat being produced in one portion of the resistor being diffused substantially throughout the whole resistor.

Still another object of the present invention is to provide an adjustable electric resistor that achieves the above objects but yet which is relatively economical to manufacture, exceedingly durable in use and consists of relatively few parts.

In the adjustable electric resistor of the present inven tion there is a base which carries an elongate element which, in the specific embodiment shown herein, is tubular. Around the exterior periphery of the tubular element a length of resistance wire is wound to form a helical coil having spaced apart turns. A slider is mounted on the base to move axially of the tubular element and the coil and carries at least one contact which is maintained in engagement with a turn or turns of a coil. Hence by moving the slider, the resistance of the adjustable resistor may accordingly be changed.

In accordance with the present invention in order to prevent changes in temperature from causing the turns to move on the periphery thus interfering with the operation of the resistor, tension is continually maintained by the tubular element on the turns to cause them to maintain their position on the element irrespective of the temperature of the resistor. This is achieved by forming the tubular element in a manner which enables it to have a variable transverse peripheral length and specifically it is formed with a slot that extends throughout the length of the element. Resilient means which in the specific embodiment shown consists of a spring but may also be inherently formed in the element is employed to continually urge the opening of the gap to cause enlargement of the peripheral length. The element is formed of metal which has a temperature coefficient of expansion that is greater than the temperature coefficient of expansion of any resistance wire wound thereon. Accordingly, when the temperature of the resistance wire and tubular element is high, the tubular element will have to increase its peripheral length greater than the length of each of the turns of the resistance wire. However, rather than stretching the turns, the difference in length is absorbed by the slot acting against the urgings of the resilient means to maintain tension on the turns.

Moreover in many instances where there are only a few turns of the resistance coil being utilized, heat is developed only at these few turns and thus is concentrated at only a localized area of the tubular element thereby causing excessive temperature thereat. The present invention by utilizing a metal of heat conducting material to form the tubular element enables heat to be efiiciently conducted from the localized area throughout the resistor.

Other features .and advantages will hereinafter appear.

In the drawing:

FIGURE 1 is a front view of the adjustable electric resistor of the present invention.

FIG. 2 is an end view thereof.

FIG. 3 is a section taken on the line 3-3 of FIG. 1.

Referring to the drawing, the adjustable electrical resistor is generally indicated by the reference numeral 10 and includes end frames 11 and 12 of sheet metal having the shape shown. The bottom of the end frames is shaped to provide supports 13 and 14 for supporting the resistor on a flat surface. Extending between the end frames to maintain them in spaced apart relation is a horizontal strut member 15 also formed of sheet metal.

Both ends of the strut are provided with a plurality of tabs 16 each of which extends through an aperture 17 formed in the end frames 11 and 12 and are bent over against the exterior surface of its associated end frame to lock the strut member to the end frames.

Adjacent the top surface of the resistor there is provided a slide bar 18 of rectangular cross-section which extends through aperture 19 in the end frames 11 and 12 respectively but is insulated therefrom as by electrical insulating bushings 21 and 21' for the end frames 11 and 12 respectively. The insulating bushings lock the slide bar to the end frames by mating shapes so that the slide bar serves to provide solidity to the top portions of the end frames. Mounted for sliding movement on the slide bar 18 is a slider 22 that includes a block 23 of electrical'insulating material, such as plastic, having the shape shown to provide a handle and also having a pair of screws 24 for securing a contact means to the slider. The contact means includes a bent resilient conductor 25 having contacts 25a mounted at each of its end portions and an elongate resilient contact spring 26 having bent end portions 27 that are resiliently urged into contact with the slide bar. The conductor 25 urges the contacts 25a towards each other in use while the spring 26 maintains electrical contact with the slide bar.

It will thus be appreciated that current may be conducted from the contacts 25a of the contact means through the conductor 25 to the spring 26 and then through the portions 27 to the slide bar. The slide bar is formed of electrical conducting metal and has end portions 18a and 18a' that project beyond their associated end frame and to each of which an exterior connection may be made.

The end frame 11 has an inwardly bent tongue 28 and similarly the end frame 12 has an inwardly bent tongue 29 with a tubular element 30 being connected thereto as by screw and bolt assemblies 31 with the screw passing through an aperture formed radially in the tubular element 30. Preferably the connection between the tongues 28 and '29 and the tubular element 30 includes electrical insulation to electrically isolate the element from the frame 11. The tubular element is formed of metal, preferably aluminum, for reasons hereinafter apparent and while shown as being circular in cross-section, may have a different cross-sectional shape if desired. Covering the exterior periphery of the element 30 is a coating 32 of a silicon base high temperature paint. The coating is shown enlarged in FIG. 3 and is preferably relatively thin, being on the order of three to five mils thick. This thickness has been found to provide electrical insulation between the turns and the element but yet enable heat to be conducted to the element 30.

Helically wound on the periphery of the tubular element 30 is a length of resistance wire 33 which in the embodiment shown is rectangular in cross-section though other wire having different cross-sectional shapes may be employed, if desired. The wire is wound so that each turn 33a is separated slightly from its adjacent turns to prevent short-circuiting therebetween and also wound in a single layer, thereby enabling each turn to be contacted by the contacts 25a of the contact means. One end 34 of the wire 32. is connected to a terminal connection, generally indicated by the reference numeral 35, that is fastened to the element 30 by a nut and bolt assembly 36. The terminal connection includes a first solderless terminal connector 37 which clamps the end 34 of the resistance wire to a somewhat L-shaped conducting bracket 38 that also has secured thereto an exterior connector 39. Similarly the other end 40 of the resistance wire 33 is connected to a terminal connection 41 similar to the terminal connection and which also includes an exterior connector 42.

In carrying out the present invention the tubular element 30 is formed of metal, such as aluminum, and is also formed with an axially extending peripheral slit 43 extending throughout its length. Bridging the slit between two abutments 44 formed interiorly of the tube, is a compressed spring 45 which may be of the shape shown, i.e. arcuate, and which preferably extends substantially throughout the length of the tube in order to apply a constant tension throughout the length of the tubular element. While a spring is shown, it will also be understood that if desired, the spring may be eliminated and the tumS wrapped about the element to compress it by deforming it from its normal shape. In either event, the compressed spring or compressed element forms resilient means that provide a tensioning on the turns to maintain them in position.

It will be appreciated from the foregoing that electrical connections may be made to theends 18a and 18a and to the terminal connections 35 and 41. Thus for example if a connection is made to the terminal connection 35 and the slide bar end 18a then the portion of the resistance wire leftward of the solid line position of the slider 22 would be placed in series between these two connections. Moreover, by varying the position of the slider 22 on the slide bar 18, the number of turns and hence the length of resistance wire in series may accordingly be varied.

It has been found that in use the portion of the resistance wire in series with the exterior connections converts electrical energy into heat and with the present invention, by reason of the structure of the tubular element, the heat is conducted from the heated turns to the element through the coating 32. As the element is made from metal having a good heat conducting characteristic, the heat is thus accordingly spread throughout the length of the element where it may be easily dissipated. Thus an excessive concentration of heat solely in the area of the tubular element where the heated turns are present is prevented. It will be appreciated that the coating 32 is thin enough to effectively electrically insulate the turns of the resistance wire from the tubular element but yet enables passage of heat from the resistance wire to the tubular element.

In making the resistor 10, the turns are initially wound on the element 30, tightly, so that when the resistor is cool, the spring 45 urges the abutments 44 apart to provide a tension on the turns. When the adjustable electrical resistor of the present invention is in use and is thus hot, the material of the tubular element 30 expands to increase its peripheral length greater than the turns of the wire by reason of their relative temperature coefiicients of expansion. However, the turns prevent expansion of the periphery greater than the inner length of the turns and the excess expansion of the tubular element is accommodated by the closing of the slot 43 against the tension of the spring 45. Thus when hot, the tubular element continually maintains a constant tension on the turns to maintain them in position but the pressure is limited by the spring 45 to a degree which prevents unwanted stretching of the turns. When the resistor has cooled after use, the tubular element will still maintain the tension on the turns by the slot 43 being enlarged by the spring 45. By the use of the spring and slot or by the compression of the element by the turns, the element 30 is thus formed to have a variable transverse peripheral length which exerts sufficient tension on the turns to maintain them in position at all temperatures of use of the resistor.

The strut 15, as shown, includes a horizontal portion 15a which is somewhat greater than the width of the element and it has been found that with this structure heat radiated from the resistance wire is prevented from effecting the surface on which the resistor 10 is placed. Thus the strut 15 not only serves as a structural element but also serves as a heat shield.

It will accordingly be appreciated that there has been disclosed an adjustable electrical resistor of the slide wire type having an elongate tubular element on the periphery of which a length of resistance wire is wound to form a coil of spaced apart turns. The tubular element in order to prevent mechanical dislocation of the turns is formed to have a variable peripheral length that together with resilient means continually applies an expanding tension on the turns irrespective of whether the resistor is cool or being used and thus hot.

Variations and modifications may be made within the scope of the claims and portions of the improvements may be used without others.

I claim:

1. An adjustable electrical resistor comprising a base, an elongate tubular element mounted on said base and formed to have a variable transverse peripheral length, a length of resistance wire wound on the periphery of said tubular element to form a coil having a plurality of electrically spaced apart turns, contact means mounted on said base for movement along said coil in engagement with at least one turn of said coil, electrical terminals connected to said coil and contact means, said element being formed of metal having a higher temperature coefiicient of expansion than the metal of the resistance wire with an increase in temperature of both tending to enlarge the peripheral length of the element greater than the peripheral length of the turns, and resilient spring means operatively associated with said tubular element for tending to enlarge the peripheral length, said turns constraining the enlargement of the peripheral length to maintain tensional engagement between the periphery of the element and the turns to hold the turns in place and in which the elongate tubular element is formed with a slot extending throughout its length and having a thickness sufficient to accommodate the difference in length between the turns and peripheral length at the operating temperature of the resistor and the resilient means tends to widen the slot and includes a compressed spring element disposed adjacent said slot and tending to enlarge the width of the slot.

2. The invention as defined in claim 1 in which a pair of transversely spaced abutments are formed on said element with there being an abutment adjacent each side of the slot, the spring is a unitary elongate piece of metal that is transversely curved and has sides that engage said abutments to span the slot and be tensioned to force said abutments away from each other.

3. The invention as defined in claim 1 in which the base includes two spaced apart frame members, with each having support means for supporting the resistor, said r tubular element being mounted between said frame members above said support means, and a plate extending between said frame members between said support means and said element and having a width greater than the element to provide a heat shield by being positioned between said support means and said element.

References Cited UNITED STATES PATENTS 1,187,228 6/1916 Asch 338-202 X 1,955,906 4/1934 Crouse 338202 X 2,416,393 2/1947 Huckleberry 336-179 2,853,587 9/1958 MacCauley 338302 RICHARD M. WOOD, Primary Examiner.

I. G. SMITH, Assistant Examiner. 

